public int minDepth(TreeNode root) {
        return dfs(root);
    }
    int dfs(TreeNode root ){
        if (root == null) return 0;
        if (root.left == null && root.right == null){
            return 1;
        }
        if (root.left != null && root.right != null){
            return Math.min(dfs(root.left)+1,dfs(root.right)+1);
        }else if (root.left != null){
            return dfs(root.left)+1;
        }else {
            return dfs(root.right)+1;
        }
    }

    public int minDepth(TreeNode root) {
        if (root == null){
            return 0;
        }
        if (root.left == null && root.right == null){
            return 1;
        }
        int min_depth = Integer.MAX_VALUE;
        if (root.left != null){
            min_depth = Math.min(min_depth,minDepth(root.left));
        }
        if (root.right != null){
            min_depth = Math.min(min_depth,minDepth(root.right));
        }
        return min_depth+1;
    }